As the need for increased data storage changes, the search for higher density, faster access memory technologies also increases. One of these, holographic data storage, provides the promise for increased access to higher density data. The techniques for realizing such storage typically utilize some type of storage media, such as photorefractive crystals or photopolymer layers, to store 3-D "stacks" of data in the form of pages of data. Typically, coherent light beams from lasers are utilized to perform the addressing, writing and reading of the data from the storage media by directing these beams at a specific region on the surface of the media. Writing is achieved by remembering the interference pattern formed by these beams at this region. Reading is achieved by detecting a reconstructed light beam as it exits the storage medium, the data then being extracted therefrom. Addressing is achieved by the positioning of the laser beams, and this is typically done through the mechanical movement of mirrors or lenses; however, the storage media itself can be moved relative to fixed laser beams.
One of the limiting aspects to the density of storage in the storage media is the physical separation between storage areas. A storage area or region is typically defined by the intersecting diameters of two beams. When these beams impinge upon a given area, the data is stored within the intersecting or overlapping area of the two beams and contained within the underlying structure of the media. The useful recording portion of the media is therefore confined to the overlap area. However, in actuality, parts of each beam will spread out beyond the overlap area. This will expose the media with useless information. Further, the overlap area may contain too much information and could be made smaller, such that it may be desirable to clip or aperture the overlap area. Therefore, some type of guard ring or region is desirable between storage regions.
Another aspect to be considered when dealing with adjacent regions in media such as photopolymer materials, is the diffusion of monomers within the media. Whenever one region is subjected to a Write operation, there will be some migration of monomers in the material, this being necessary to obtain contrast in the recording. Ideally, it would be desirable to locally confine this migration to the area of the recording. However, since the material is relatively homogenous and all regions are contiguous, migration of monomers can occur between recording regions.